1. Field of the Invention
This invention relates generally to integrated circuits and, more particularly, to a tunable constant current source with temperature and power supply compensation.
2. Discussion of the Related Art
The generation of a constant current or constant voltage is one of the most critical portions of any analog integrated circuit. Upon generating the constant current or constant voltage source, all other analog circuits are biased off this constant source within the integrated circuit itself. Historically, a constant current source has been supplied by use of a band-gap reference network that is formed by a diode and resistor to generate a constant current source.
While a band-gap reference network works well at conventional supply voltages, such as 5 volts (VCC), as power supply voltages decrease, the band-gap implementation becomes ineffective to inoperative. This is due to the diode drop which is between about 0.7 to 0.8 volts which does not scale and leaves less headroom for current source cascading. In most instances, a band-gap reference network will work well down to a supply voltage of about 2.5 volts and will operate with significant jitter down to about 1.8 volts. However, with new lower voltage power supplies operating in a range of about 1.5 volts, the band-gap reference network is not a viable option. Additionally, the band-gap reference network has undesirable power supply tracking characteristics. These characteristics cause excessive error with respect to power supply noise (delta in output due to delta in power supply).
Since constant current sources are generally used for phase lock loops (PLL) which are generally used to generate clocks for microprocessors, the above disadvantages have drastic effects on microprocessor operation. Moreover, since about 95% of all microprocessors use phase lock loops for generating clock pulses, this effects a broad range of microprocessors operating today. Thus, low power supply microprocessors may not use band-gap reference networks to provide constant current sources. In addition, because of the high sensitivity by band-gap reference networks to power supply changes, this creates jitter in the phase lock loop which adds directly to the cycle time of the processor, thereby reducing the operating speed and efficiency of the microprocessor.
What is needed then is a tunable constant current source with temperature and power supply compensation that does not suffer from the above-mentioned disadvantages. This will, in turn, provide a current source that has a very low sensitivity to power supply changes, provide a low clock skew and jitter in a phase lock loop, thereby reducing cycle time so that the microprocessor may run faster, provide operating supply voltage ranges between about 0.5 volts to about 1.8 volts, provide a constant current within an integrated circuit that can be used for any analog application, such as a phase lock loop which is used for processor clock generation, and provide a constant current source that compensates for temperature variations efficiently. It is, therefore, an object of the present invention to provide such a tunable constant current source with temperature and power supply compensation in an integrated circuit.